Equivalent Focal Length of Lenses in Contact (1/f = 1/f₁ + 1/f₂)
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1/f = 1/f₁ + 1/f₂
What each symbol means
| Symbol | What it stands for |
|---|---|
| f_1 | Focal length of first lens (with sign) |
| f_2 | Focal length of second lens (with sign) |
| f_{eq} | Equivalent focal length of the combination |
When to use this
Thin lenses in contact. P_eq = P₁ + P₂ (powers add). f can be negative (diverging combination). Extends to N lenses: 1/f = Σ(1/fᵢ). This entry emphasizes power addition and sign handling.